1. Field of the Invention
The present invention relates to a sheet-stacking apparatus configured to continuously stack a large number of sheets while sheets already stacked are being removed and, more particularly, to an apparatus effectively using a sheet-stacking space and an image-forming apparatus having such a sheet-stacking apparatus.
2. Description of the Related Art
In recent years, together with the technological advancement in image-forming techniques, an image-forming apparatus configured to form an image on a sheet has become capable of discharging sheets from its main body in larger quantities at higher speed. Accordingly, a sheet-stacking apparatus which is connected to a main body of the image-forming apparatus and stacks sheets discharged from the apparatus main body is required to be capable of stacking a large number of sheets. A sheet-stacking apparatus satisfying such a request (hereinafter referred to as “stacker apparatus”) is discussed in Japanese Patent Application Laid-Open No. 2006-124052.
This conventional stacker apparatus is shown in FIG. 21. A stacker apparatus 500 receives a sheet discharged from a main body of an image-forming apparatus at an inlet roller 501 and then passes the sheet over to a gripper 503 using a conveyance roller pair 502. The gripper 503 holds and conveys the sheet so that the leading edge of the sheet abuts against a leading edge stopper 504. At the leading edge stopper 504, the sheet is released from the gripper 503 and falls onto a stacker tray 505. At this time, the sheet falls between the leading edge stopper 504 and a trailing edge stopper 508. Then, the leading edge and the trailing edge of the sheet on the stacker tray 505 are aligned. Further, if necessary, the sheet is aligned by a width alignment device (not shown) in the width direction which is perpendicular to the sheet-conveyance direction so that the side end of the sheet is aligned. Furthermore, with consideration given to a case when the sheet is curled or when there is a space between sheets, the sheets are pressed against the stacker tray 505 by a leading edge pressing member 506 and a trailing edge pressing member 507 in every predetermined number of sheets so that the stacked sheets do not interfere with the subsequent sheet.
The conventional stacker apparatus has only one stacker tray 505 whose size is adjusted to a maximum length of a sheet. Accordingly, even when a small sheet is set (for example, B5 size) and two sheet stacks can be arranged side-by-side on the stacker tray 505, only one sheet stack is possible. Accordingly, when a small sheet is stacked, the conventional stacker apparatus has an empty space X within the stacker apparatus. Thus, the space in the conventional stacker apparatus is not efficiently used. Further, the conventional stacker apparatus has to be stopped while a user removes the sheets, thus a large number of sheets cannot be continuously stacked.
Accordingly, a conventional image-forming apparatus equipped with such a stacker apparatus is unable to continuously stack a large number of sheets and has low efficiency in image forming.